In recent years, there has been developed an electromechanical power generating device (power generator) which can convert mechanical vibration energy into electric energy (for example, see patent document 1). An electromechanical power generating device disclosed in the patent document 1 has one resonance frequency. From the view point of efficiently generating electric power with a small space, it may be considered to carry out power generation with this electromechanical power generating device by utilizing vibration of an air-conditioning duct, for example.
In this regard, the air-conditioning duct has its own natural frequency. Due to an action of the natural frequency, plate materials forming the air-conditioning duct are always vibrated in a direction substantially perpendicular to a flowing direction of fluid (air) during air-conditioning. Examples of a vibration source for this vibration of the plate materials include a pressure difference (pulse beat) of the fluid flowing in the duct, variation of swirl or density of the fluid caused by an enforced changing of the flowing direction of the fluid in the duct and motion vibration caused by a walking of a person or a working of an apparatus and transmitted from the periphery of the duct.
However, when a distribution of a vibration frequency of the duct is shown in a graphic form, it is a rare case that the distribution of the vibration frequency of the duct is expressed as a spectrum form having only one frequency. Generally, the distribution of the vibration frequency of the duct generally has a various spectrum forms depending on the type of duct. Namely, the distribution of the vibration frequency significantly varies according to the type of duct. Since the power generating device disclosed in the patent document 1 has only one resonance frequency as described above, the power generating device utilizes only a part of the vibration frequency of the duct which is distributed over a broad frequency band. In this case, the power generating device cannot efficiently generate the electric power. Further, in the case where the one resonance frequency of the power generating device is significantly different from a main part of the distribution the vibration frequency of the duct, the power generating device cannot efficiently generate the electric power.